"Genesis of the Grail Kings" is a book that belongs to a very specific niche of works which have become more popular with every year. Perhaps there is a core of former hippie-seeker-Baby-Boomers that are driving the sales and writing of these works--like the oft described panic of the near-retirement-age investors driving up the NASDAQ. If you have been a questioner much of your life and take the axiom to an extreme that says the unexamined life is not worth living, then Mr. Gardner's work may well fascinate and entertain.
Mr. Gardner has a very readable, if not somewhat cliche-dotted style. He relates his ideas like a story, not like a formal treatise. While he footnotes generously, his writing is accessible rather than tedious, as are some historical analyses of ancient civilizations.
So much of what is known about pre-Biblical Middle Eastern culture has remained the purview of theological and academic research that it is difficult to know to what extent Mr. Gardner's assertions are reasonable without taking on a similarly arduous project to his own. I found the subject fascinating, thought provoking, and far-fetched, but not irrational.
Read the book as an intellectual exercise and decide for yourself.